Offsets and low frequency noise, also referred to as 1/f noise or pink noise, are often undesirable in electronic circuits. One solution that has been applied to remove offsets and 1/f noise is calibration. Another method is auto-zeroing for which the offsets and 1/f noise are sampled and placed in series with the input signal but with opposite polarity. Another method is correlated double sampling in which two samples are collected and the first is subtracted from the second. This method introduces a high pass filter that must be acceptable in the given application. Another approach to mitigating the effects of offsets and 1/f noise is referred to as chopping, a process by which offsets and noise are modulated to a higher frequency.
FIG. 1 is a block diagram illustrating a system 100 that includes a Hall sensor circuit and a spinning current modulation technique (SCMT) apparatus. FIG. 1, in essence, was originally presented as FIG. 1 in the paper by Vincent Mosser, Nicolas Matringe, and Youcef Haddab, “A Spinning Current Circuit for Hall Measurements Down to the Nanotesla Range”, IEEE Trans. Instrum. Meas., vol. 66, no. 4, April 2017, referred to hereafter as “the SCMT paper,” which is hereby incorporated by reference in its entirety for all purposes.
The system 100 includes an analog-to-digital converter (ADC) 116 (e.g., a sample-and-hold and ADC), a computer (“PC”) 122, and a microcontroller 118 coupled to the computer 122. The microcontroller 118 performs the SCMT by generating clock signals C1-C9. Clock signals C1-C8 control a set of switches 106, and C9 controls the sampling rate of the ADC 116. The microcontroller 118 also performs digital demodulation. The switches 106 are coupled to a current source 102, a pre-amplifier 112, and a Hall plate 104. The Hall plate 104 is in the shape of a cross with a contact designated a+ at the north position, a contact designated a− at the south position, a contact designated b+ at the east position, and a contact designated b− at the west position, as shown. The four contacts of the Hall plate 104 are coupled to the switches 106 by an interconnect cable 108. The current source 102 has two contacts denoted I+ and I− also coupled to the switches 106. The pre-amplifier 112 measures a voltage on input contacts denoted V+ and V− that are coupled to the switches 106. The pre-amplifier 112 amplifies the measured voltage and provides the amplified voltage to an antialiasing filter 114. The filter 114 provides the filtered voltage to the ADC 116 which samples the filtered voltage and provided digital voltage values to the microcontroller 118.
The switches 106 include sixteen (16) switches that are connected to the four contacts of Hall plate 104, the two contacts of current source 102, and the two contacts of pre-amplifier 112 in such a manner as to enable eight possible ways to connect the current source 102 to the Hall plate 104 and to connect the pre-amplifier 112 to usefully measure a voltage produced by the Hall sensor 104 in response to the current forced into it by the current source 102, as shown in the table of FIG. 2, which is discussed in more detail below. This configuration enables the microcontroller 118 to control the switches 106 via the clocks C1-C8 to perform 2-dimensional chopping, which is commonly referred to as “spinning,” or SCMT in the SCMT paper. The SCMT is performed on offset and noise sources, which include Hall sensor offset and 1/f noise, preamplifier (PA) offset and 1/f noise, and pickup and electromagnetic field (EMF) voltages. The microcontroller 118 controls the sixteen switches 106 via the eight clock signals C1-C8 to perform the 2-dimensional chopping. The filter 114 is not meant to remove the chopped signal, but instead performs an antialiasing function.
FIG. 2 is a table 200 that includes eight rows that correspond to eight possible configurations 1-8 to connect the current source 102 contacts I+ and I− and the pre-amplifier 112 contacts V+ and V− to the Hall plate 104 contacts a+, a−, b+, and b− of FIG. 1 that result in a useful measurement of the voltage measured by the pre-amplifier 112. The left-hand portion of table 200 includes four respective columns corresponding to contacts I+, I−, V+, and V−. Each column/row cell indicates the Hall plate 104 contact (a+, a−, b+, b−) connected to the corresponding column contact. The right-hand portion of table 200 includes four respective columns corresponding to the true/desired Hall signal, the Hall sensor offset and 1/f noise, the PA offset and 1/f noise, and the pickup and EMF voltages. The four columns indicate the resulting voltage polarity for each of the eight configurations 1-8.
FIG. 3 is a table 300 that illustrates a manner in which the microcontroller 118 of FIG. 1 may chop the various offsets and 1/f noise to a higher frequency by choosing various combinations of two pairs of configurations 1-8 of table 200 of FIG. 2 and cycling through them repeatedly, as described in more detail in the SCMT paper. More specifically, table 300 shows four rows corresponding to configurations 3-6 of FIG. 2. Four columns in the left-hand portion of table 300 indicate the connections of the contacts for the current source 102, pre-amplifier 112, and Hall plate 104 corresponding to configurations 3-6, similar to FIG. 2. Four columns in the middle portion of table 300 indicate the resulting voltage polarity of the desired Hall signal, the Hall sensor offset and 1/f noise, the PA offset and 1/f noise, and the pickup and EMF voltages for each of configurations 3-6, similar to the right-hand portion of table 200 of FIG. 2. In the right-hand portion of table 300, a column indicates a sign, i.e., +1 or −1, of demodulation performed by the microcontroller 118 on the digital filtered voltage received from the ADC 116 as necessary, e.g., for configurations 5 and 6 of table 300 to make the polarity of the Hall signal positive. Finally, in the far right-hand portion of table 300, four columns similar to the middle four columns of table 300 indicate the resulting voltage polarity of the desired Hall signal, the Hall sensor offset and 1/f noise, the PA offset and 1/f noise, and the pickup and EMF voltages after the demodulation.
It may be observed from tables 200 and 300 that two pairs of connections are required to accomplish the chopping/spinning because no single pair of connections can be chosen that will chop all of the offsets and noise; thus, the need for 2-dimensional chopping, or spinning. It may also be observed from tables 200 and 300 that some of the connections require the chopped signal to be inverted to restore the Hall signal.
FIG. 4A is a frequency spectrum graph representation of a signal generated by a Hall sensor device. FIG. 4A shows the desired Hall signal low in the frequency spectrum and shows offsets and 1/f noise also produced by the Hall sensor device even lower in the frequency spectrum.
FIG. 4B is a frequency spectrum graph representation of a result of 2-dimensional chopping of the signal of FIG. 4A generated by the Hall sensor device, more specifically of a result of chopping of the offsets and 1/f noise of FIG. 4A. A potential disadvantage of chopping is that for a sample rate of fs, the 2-dimensional chopping produces a tone at fs/2 and fs/4. As shown in FIG. 4B, the desired Hall signal is low in the frequency spectrum, and the chopped offsets and noise appear as two tones (e.g., at fs/2 and fs/4) higher in the frequency spectrum. In some applications, the tones introduced by chopping may not be acceptable. More generally, for a system that employs n-dimensional chopping, the lowest produced tone will be at fs/2n. That is, for each additional dimension, chopping introduces an additional tone at a frequency that is ½ the previous frequency. As the chopping dimensionality increases, the tones rapidly decrease in frequency due to the exponential nature of the lowest frequency tone produced. Keeping the low frequency tones out of the band of interest may force the chopping frequency to be set by the lowest frequency tone, which may be undesirable. Finally, if any offset remains after chopping, a signal must be fed back in order to cancel the remaining offset, which adds noise to the system.